System and method for detecting presence of digitally modulated waveform

ABSTRACT

The disclosure is directed to a system for detecting digitally modulated waveforms on a transmission channel. The system includes an amplitude modulation detector configured to detect amplitude variations on the transmission channel. The system also includes a first bandpass filter configured to receive a signal from the amplitude modulation detector monitoring the transmission channel and tuned to a wavelength corresponding to the symbol rate of the digitally modulated waveform to be detected. Further, the system includes a second bandpass filter configured to receive the signal from the amplitude modulation detector monitoring the transmission channel and tuned to a second wavelength. Further still, the system includes a means for comparing the output of the first and second bandpass filters to determine the presence of a digitally modulated waveform.

BACKGROUND OF THE INVENTION

The present application relates generally to the field of datalinkcommunications. More particularly, the application relates to a systemand method to detect the presence of a digitally modulated waveform on atransmission channel.

Very High Frequency Digital Link (VDL) Mode 2 is a type of datalinkcommunication system used in Aeronautical communication systems. VDLMode 2 is used to improve air traffic management, leading to improvedoperational efficiency among other improvements, in the aircraftindustry.

VDL Mode 2 is a carrier sense multiple access (CSMA) packet data system.CSMA is an access technique that requires a user check the transportmedium using a listen function prior to transmitting. If the transportmedium is busy, the user is required to hold for a period of time priorto reinitiating a transmitting function. If two stations transmitsimultaneously on the transport medium, the resultant collision cancorrupt both messages. Further, the physical layer of VDL Mode 2 furtherutilizes a 31.5 kilobits per second data rate Differential Phase ShiftKeying (D8PSK) digital waveform using raised cosine filtering with 0.6excess bandwidth.

Accordingly, VDL Mode 2 allows more available bandwidth by providing afaster and more reliable transmission speed while also enhancingcommunications with flight controllers and pilots.

However, to implement the CSMA access technique, it is necessary toprevent unintentional transmissions by determining channel activity. Toprevent unintentional transmissions, the energy level of the channel ischecked in decibels (dBm) or microvolts. An energy level threshold, usedto prevent the collision of messages, has been set by the RadioTechnical Commission for Aeronautics (RTCA). If the channel is above acertain energy level threshold, currently approximately −87 dBm, thechannel is considered busy and the user must delay transmitting untilthe channel is clear. If the channel energy level is below thethreshold, the user is free to transmit.

Traditionally, the energy level of the channel has been monitored byexamining the automatic gain control (AGC) level of a receivermonitoring the channel. The AGC level is determined by monitoring theautomatic gain control response on a receiver. However, the AGC level isresponsive to any kind of energy on the channel, whether the source is aVDL Mode 2 signal, an unmodulated carrier, or other interference.Accordingly, monitoring the AGC level can create many false positiveswhen detecting the presence of a VDL Mode 2 signal. False positives candegrade network efficiency and cause system indications to an operatorof no communications.

Therefore, there is a need for a system and method for monitoring acommunication channel and to distinguish a desired type of modulatedwaveform from ambient interference sources and to prevent falsepositives. There is also a need for a system and method for monitoring acommunication channel carrying VDL Mode 2 signals to detect the presenceof VDL Mode 2 signals and distinguish the signals from ambientinterference sources to prevent false positives.

SUMMARY OF THE INVENTION

One exemplary embodiment of the invention relates to a system fordetecting the presence of a digitally modulated waveform on atransmission channel. The system includes an amplitude modulationdetector configured to detect a change in the energy level of thetransmission channel, a first bandpass filter configured to receive asignal from the transmission channel and tuned to a wavelengthcorresponding to the symbol rate of the digitally modulated waveform tobe detected, a second bandpass filter configured to receive the signalfrom the transmission channel and tuned to a second wavelength, andmeans for comparing the output of the first and second bandpass filtersto determine the presence of a digitally modulated waveform.

Another exemplary embodiment of the invention relates to a VHFcommunications transceiver having an improved transmission throughput.The communications transceiver includes a first antenna for monitoring atransmission channel, a second antenna for sending a signal over atransmission channel, and a system for detecting the presence of adigitally modulated waveform on the transmission channel. The systemincludes an amplitude modulation detector configured to detect a changein the energy level of the transmission channel, a first bandpass filterconfigured to receive a signal from the transmission channel and tunedto a wavelength corresponding to the symbol rate of the digitallymodulated waveform to be detected, a second bandpass filter configuredto receive the signal from the transmission channel and tuned to asecond wavelength, and means for comparing the output of the first andsecond bandpass filters to determine the presence of a digitallymodulated waveform.

Yet, another exemplary embodiment of the invention relates to a methodfor detecting the presence of a digitally modulated waveform on atransmission channel prior to transmitting a signal. The method includesmonitoring a transmission channel to detect a change in energy level,performing a first filtering of a signal from the transmission channelat a first wavelength corresponding to the symbol rate of the digitallymodulated waveform to be detected, performing a second filtering of thesignal from the transmission channel at a second wavelength to provide acontrol value, and comparing the output from the first and secondfiltering to determine the presence of a digitally modulated waveform.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingdrawings, wherein like reference numerals refer to like elements, and inwhich:

FIG. 1 is a schematic diagram illustrating a system 100 for detectingthe presence of a VDL Mode 2 signal according to an exemplaryembodiment;

FIG. 2 is a graph showing the spectral line found in the VDL Mode 2waveform at 10.5 kHz; and

FIG. 3 is a flowchart illustrating a method for detecting the presenceof a VDL Mode 2 waveform.

DETAILED DESCRIPTION OF THE-PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram illustrating a system 100 for detectingthe presence of a VDL Mode 2 signal according to an exemplaryembodiment. The system can include an amplitude modulated(AM) detector110, a first bandpass filter 120, a second bandpass filter 130, a firstenergy detector 140, a second energy detector 150, an energy comparecomponent 160, and a signal presence indicator 170.

According to an exemplary embodiment, AM detector 110 can be an envelopedetector. An envelope detector can be used to measure or detect theamplitude (as opposed to the power or phase) of a digitally modulatedwaveform. According to an exemplary embodiment, AM detector 110 can be ahalfwave rectifier which charges a capacitor to a voltage approximatelyequal to the peak voltage of the incoming AM waveform. When the inputwave's amplitude increases, the capacitor voltage is increased via therectifying diode. When the input's amplitude falls, the capacitorvoltage is reduced by being discharged by a ‘bleed’ resistor.

AM detector 110 is used to detect the energy level on the transmissionchannel. Wherein the energy level is detected to have an energy levelhigher than the threshold level set by transmission system protocols, adetermination must be made whether a VDL Mode 2 signal is present orwhether the increased energy level is caused by interference.

Accordingly, prior to transmitting, the signal on the transmission lineis provided as uniform inputs to first bandpass filter 120 and secondbandpass filer 130. First bandpass filter 120 and second bandpass filter130 can be electronic devices or circuits that allow signals between twospecific frequencies to pass, but discriminate against signals at otherfrequencies. The main function of such a filter in a transmitter is tolimit the bandwidth of the output signal to the minimum necessary toconvey data at the desired speed and in the desired form. In a receiver,a bandpass filter allows signals within a selected range of frequenciesto be heard or decoded, while preventing signals at unwanted frequenciesfrom getting through. A bandpass filter can also optimize thesignal-to-noise ratio (sensitivity) of a receiver.

According to an exemplary embodiment, first bandpass filter 120 can betuned to 10.5 kHz and second bandpass filter 130 can be tuned to 9.5kHz. Although 9.5 kHz is described in the exemplary embodiment, secondbandpass filter 130 can be tuned to any adjacent band near the bandprescribed for the first bandpass filter. The 3 dB bandwidth of eachfilter is a nominal 500 Hz with a nominal 20 dB rejection and +/−1000Hz. The 3 dB bandwidth was chosen to minimize time delay through thefilter consistent with detection times and ability to reject adjacentbandpass filter energy.

According to an exemplary embodiment, 10.5 kHz can be chosen as thefilter band for first band based on the existence of a detectablespectral line at that bandwidth wherein a VDL Mode 2 signal is detected.The spectral line correlates to the natural symbol rate found in thesignal. The natural symbol rate is the rate at which data is encoded fortransmission. This spectral line is present on the output of an AMdemodulator. It is caused by an inherent amplitude modulation componentof the D8PSK waveform. The spectral line will not be present in phase orfrequency demodulation of the D8PSK waveform, the demodulation methodused to recover message content.

The output from first bandpass filter 120 can be provided to firstenergy detector 140 and the output from second bandpass filter 130 canbe provided to second energy detector 150 according to an exemplaryembodiment. The first and second energy detectors can be used todetermine the energy level of the signal received from the first andsecond bandpass filter.

The output from first energy detector 140 and second energy detector 150is provided as inputs to energy compare 160. Energy compare 160 can beany form of level comparator, such as but not limited to an operationalamplifier or other specialized equipment such as dual analog-to-digitalconverters coupled to a processor, to compare levels. According to anexemplary embodiment, the energy output from energy detector 140 iscompared to the energy output from energy detector 150 as a ratio. Ifthe ratio of the outputs exceeds a preset level, presence of a VDL Mode2 signal is indicated using signal presence indicator 170. Signalpresence indicator 170 can be a data instruction, a mechanicalindicator, a signal or any other type of indicia for communicating thepresence of a VDL Mode 2 signal.

In operation, according to an exemplary embodiment, VDL Mode 2 signalsinclude a low detectable spectral line at 10.5 KHz. When a VDL Mode 2signal is present, the energy level output from the first bandpassfilter (e.g. filter 120), will be higher in comparison to the energylevel output from the second bandpass filter (e.g. filter 130). Theresultant ratio will indicate the presence of a VDL Mode 2 signal andsignal presence indicator 170 can be used to indicate the ability toinitiate transmission.

If the monitored energy level on the channel changes as a result ofnoise, there will be minimal, if any, difference between the energylevel measured from first bandpass filter 120 and second bandpass filter130. Accordingly, no VDL Mode 2 signal is detected and a new VDL Mode 2signal can be transmitted with reduced chance for collision andcorruption of the signal.

FIG. 2A is a graph 200 showing a spectral readout of the energy outputfrom an AM detector when presented with a pseudo-random combination ofD8PSK data. A horizontal axis 210 shows frequency from 0 Hz to 15 KHz. Avertical axis 220 indicates relative amplitude. A spectral line ofenergy 230 is displayed in the graph. Spectral line of energy 230 showsa spike at the approximately 10.5 kHz band 240, indicative of thepresence of a VDL Mode 2 waveform.

FIG. 2B is also a graph 300 showing a spectral readout of the energyoutput from a FM/PM detector when presented with a pseudo-randomcombination of D8PSK data. A horizontal axis 310 shows frequency from 0Hz to 15 KHz. A vertical axis 320 indicates relative amplitude. Aspectral line of energy 330 is displayed in the graph. Spectral line ofenergy 330 does not show any spike at the approximately 10.5 kHz band240 or other indication to indicate of the presence of a VDL Mode 2waveform.

FIG. 3 is a flowchart illustrating a method for sending a VDL Mode 2signal after detecting the presence of a pre-existing VDL Mode 2 signalusing the system described above with reference to FIG. 1 according toan exemplary embodiment.

Prior to sending a VDL Mode 2 signal, the transmission channel mustfirst be monitored to detect the presence of a preexisting VDL Mode 2signal. The transmission channel can be monitored using a receiverincluding automatic gain control (step 300). Automatic gain controlfunctions to indicate the presence of energy on the transmissionchannel. If energy is detected on the channel, the signal on the channelcan be sampled to determine if the energy is the result of a VDL Mode 2signal or of an interference noise.

The signal from the channel can be provided to first and second bandpassfilters (steps 310 a and 310 b). The signal is provided to a firstbandpass filter tuned to 10.5 kHz according to an exemplary embodiment(step 310 a). The first bandpass filter can be tuned to 10.5 kHz basedon the empirical knowledge that a VDL Mode 2 signal will cause aspectral response at 10.5 kHz based on the symbol rate of the signal.According to alternative embodiments, where the system can be used todetect the presence of other types of digitally modulated signals, thefirst bandpass filter can be tuned according to the symbol rate of thosesignals.

The signal is provided to a second bandpass filter (step 310 b). Thesecond bandpass filter can be tuned to 9.5 kHz according to an exemplaryembodiment. According to alternative embodiments, the second bandpassfilter can be set to any wavelength wherein a signal is detected but notaffected by the spectral response near 10.5 kHz or the symbol rate ofother types of digitally modulated signals. The second bandpass filtercan provide a baseline energy level measurement for comparison with thefirst bandpass filter.

The output from steps 310 a and 310 b can be measured to determine theenergy level of the signal after passing through the first and secondbandpass filters (steps 320 a and 320 b).

The output of steps 320 a and 320 b can be used to compose a ratio (step330). The ratio will compare the energy level of the signal at awavelength where a spectral response is expected if a VDL Mode 2 signalis present to the energy level of the signal at a wavelength where novariation based on the presence of a VDL Mode 2 signal is expected.

The ratio is compared to a predetermined threshold ratio to determinethe presence of a signal (step 340). If no signal is present, the energylevel of the signal at a wavelength, where a spectral response isexpected, can be approximately equivalent to the energy level of thesignal at a wavelength where there is no variation based on the presenceof a VDL Mode 2 signal. If a VDL Mode 2 signal is present, the energylevel of the signal at a wavelength where a spectral response isexpected can be greater than the energy level of the signal at awavelength where there is no variation based on the presence of a VDLMode 2 signal.

A determination is made based on step 340 whether a VDL Mode 2 signal ispresent (step 350). If a signal is not detected, a new VDL Mode 2 signalcan be transmitted (step 360). If a signal is detected, the method cancycle back to perform step 310.

While the exemplary embodiments illustrated in the FIGURES and describedabove are presently preferred, it should be understood that theseembodiments are offered by way of example only. For example, alternativeembodiments include using the above described system and method withother types of digitally modulated waveforms. According to anotheralternative embodiment the bandpass filters can be tuned to correspondwith a different symbol rate. Further, the methods disclosed may beperformed in any of a variety of sequence of steps. Accordingly, thepresent invention is not limited to a particular embodiment, but extendsto various modifications that nevertheless fall within the scope of theappended claims.

1. A system for detecting the presence of a digitally modulated waveformon a transmission channel, comprising: an amplitude modulation detectorconfigured to detect amplitude variations on the transmission channel; afirst filter configured to receive a signal from the amplitudemodulation detector monitoring the transmission channel and tuned to afrequency corresponding to a symbol rate of the digitally modulatedwaveform to be detected; a second filter configured to receive thesignal from the amplitude modulation detector monitoring thetransmission channel and tuned to a second frequency; and means forcomparing output of the first and second filters to determine thepresence of a digitally modulated waveform.
 2. The system of claim 1,wherein the digitally modulated signal is a VDL Mode 2 signal.
 3. Thesystem of claim 2, wherein the first filter is tuned to approximately10.5 kHz.
 4. The system of claim 3, wherein the second filter is tunedto approximately 9.5 kHz.
 5. The system of claim 1, wherein the firstand second filters are bandpass filter and wherein the means forcomparing the output of the first and second bandpass filters todetermine the presence of a digitally modulated waveform includes firstand second energy level detectors.
 6. The system of claim 5, wherein themeans for comparing the output of the first and second bandpass filtersto determine the presence of a digitally modulated waveform furtherincludes a means for generating a ratio comparing the output of thefirst bandpass filter to the output of the second bandpass filter. 7.The system of claim 1, wherein the first and second filters are bandpassfilter and wherein the means for comparing the output of the first andsecond bandpass filters to determine the presence of a digitallymodulated waveform includes first and second analog-to-digitalconverters coupled to a processor.
 8. A VHF communications transceiverhaving an improved transmission throughput, comprising: a first antennafor monitoring a transmission channel; a second antenna for sending asignal over a transmission channel; an amplitude modulation detectorconfigured to detect a change in an energy level of the transmissionchannel; a first bandpass filter configured to receive a signal from theamplitude modulation detector monitoring the transmission channel andtuned to a frequency corresponding to a symbol rate of the digitallymodulated waveform to be detected; a second bandpass filter configuredto receive the signal from the amplitude modulation detector monitoringthe transmission channel and tuned to a second frequency; and a computerconfigured to compare output of the first and second bandpass filters todetermine the presence of a digitally modulated waveform.
 9. The VHFcommunications transceiver of claim 8, wherein the digitally modulatedsignal is a VDL Mode 2 signal.
 10. The VHF communications transceiver ofclaim 9, wherein the first bandpass filter is tuned to approximately10.5 kHz.
 11. The VHF communications transceiver of claim 10, whereinthe second bandpass filter is tuned to approximately 9.5 kHz.
 12. TheVHF communications transceiver of claim 8, wherein the means forcomparing the output of the first and second bandpass filters todetermine the presence of a digitally modulated waveform includes firstand second energy level detectors.
 13. The VHF communicationstransceiver of claim 12, wherein the means for comparing the output ofthe first and second bandpass filters to determine the presence of adigitally modulated waveform further includes a means for generating aratio comparing the output of the first bandpass filter to the output ofthe second bandpass filter.
 14. The VHF communications transceiver ofclaim 8, wherein the means for comparing the output of the first andsecond bandpass filters to determine the presence of a digitallymodulated waveform includes providing the output of the first and secondbandpass filters respectively into first and second analog-to digitalconverters coupled to a processor.
 15. A method for detecting thepresence of a digitally modulated waveform on a transmission channelprior to transmitting a signal, comprising: monitoring a transmissionchannel to detect a change in energy level; performing a first filteringof a signal from an amplitude modulation detector monitoring thetransmission channel at a first frequency corresponding to a symbol rateof the digitally modulated waveform to be detected; performing a secondfiltering of the signal from the amplitude modulation detectormonitoring the transmission channel at a second frequency to provide acontrol value; and comparing output from the first and second filteringto determine a presence of a digitally modulated waveform.
 16. Themethod of claim 15, wherein the digitally modulated waveform is a VDLMode 2 signal.
 17. The method of claim 16, wherein the first filteringis performed at 10.5 kHz.
 18. The method of claim 17, wherein the secondfiltering is performed at 9.5 kHz.
 19. The method of claim 15, whereinthe step of comparing the output from the first and second filtering todetermine the presence of a digitally modulated waveform includes:providing output from the first filtering to a first analog-to-digitalconverter coupled to a processor, and providing output from the secondfiltering to a second analog-to-digital converter coupled to theprocessor.
 20. The method of claim 15, further comprising providing asignal on the transmission channel based on the comparison of the firstand second filtering.